The present invention relates to a wear indicator for vehicle tyres.
The grooving of tyres has an essential impact on the grip of the vehicle in dry, and particularly in wet road conditions. From the point of view of safe driving, it is important to monitor wearing of tyres and reduction of the groove depth so that a groove which has worn down and become too low would not cause a surprise in poor road conditions. Also a given groove depth in the tyres is required for a car to be in appropriate condition for inspection.
For following up the wear of a tyre, various tyre structures have been developed:
In patent specification WO-97/47483 a tyre structure is known the wearing of which can be followed by means of wear indicators disposed in the tread, and therethrough learn when the location of a tyre in the vehicle should be changed. The wear indicators are recesses formed in the tread, extending to different depths or protruding parts formed to be in conjunction with the grooves and extending to different depths. The wear indicators are spread on the entire circumference of the tyre. The depths of consecutive indicators will grow uniformly from the lowest to the deepest. When the wear of a tyre has been continuing so long that some of the indicators have worn off and the pattern formed by the indicators has therefore changed, we know that the location of the tyre should be changed. The wear indicators do not indicate anything of the groove depth.
In patent specification No. DE-OS-36 27 832, a tyre structure is known the wearing of which can be followed on the basis of the wearing of a stepped structure formed on the tread, preferably to be in conjunction with grooves. A kind of marking is made on the step levels of the stepped structure, for instance unending numbering.
However, the numbering of the step levels will not indicate anything of the depth of the groove. For instance, when steps 3 and 4 are visible in the tread, it is possible to conclude on the basis of the lowermost number xe2x80x9c4xe2x80x9d that the tread had included four steps altogether, but what this means relative to the overall wear and how much of the groove depth still remains will not become apparent.
In patent specification No. U.S. Pat. No. 2,706,509, a tyre structure is known in which wear indicators have been formed on one wall or on the opposite walls of a groove. The indicators are step-wise formed so that each step is of equal height and extends from the tread to the bottom of the-groove. The remaining groove depth is equal to the number of the remaining intact steps multiplied by the depth of the step plus the remaining depth of the step in the tread. The depth of one step or the number of the steps have not, however, been visibly marked in the steps. Because of the soiling of the tyre and the grooves being filled with soil it is difficult to examine the steps. For instance, the numbers in FIG. 5, which belong to different steps, are not measurement values but merely reference numbers of steps. When the tread extends to line 17, nine step surfaces 1 to 9 are remaining. Assuming that not a single step surface has worn off, each step height is equal to one tenth of the entire groove depth. Hence, nine full tenth parts of the groove depths are left plus one partly worn topmost step. Consequently, the wearing is something between zero and one tenth.
In patent specification No. EP-250,113 A3, tyre wear indicators are known which are formed from capillary slits of different depths and assembled into groups which have been divided across the tread on different sides of the middle line in circumference direction. Within a group, the capillary slits are mutually of the equal length. The function of the indicators is to indicate the uneven wearing of the tyre. This is carried out so that when a tyre is balanced and is wearing down evenly on both sides of the middle line in circumference direction, the tread pattern formed by the indicator groups also changes symmetrically, that is, the shortest capillary slits, mutually of equal depth, on both sides of the middle line, disappear simultaneously from the tread. When a tyre wears unevenly, the tread pattern starts to change on the side of the tyre which wears more before the tread pattern on the other side of the tyre changes. The wear indicators do not yield any information on the amount of reduction of the groove depth.
The objective of the present invention is to produce a tyre structure which is provided with indicators indicating the groove depth, on the basis of which the groove depth of the tread can easily be estimated at each time, i.e. at the moment in question.
This is accomplished with a tyre having the features described below.
In the surface part of tyre tread pattern pieces or of ribs, which are in contact with the road surface, indicators are formed in the form of sipes or grooves, extending radially into the tyre. The cross-section in the direction of the tread of the sipe represents a number informing of the groove depth of the tread in the plane in the tread direction tangential to the bottom of said indicator. Thus, the visible cross-sectional shape of the sipe indicates the groove depth in the plane of the sipe bottom as a numerical value.
According to an advantageous embodiment of the invention, the indicators form at least one group of indicators. The depth of each indicator belonging to an indicator group increases by predetermined steps, so that, respectively, the groove depth in the bottom plane of the indicator is smallest below the deepest capillary groove, growing therefrom by equal steps towards the greatest depth. The informing cross-section diagrams of the indicator group represent, for instance in numbers 2, 4, 6, 8, 10 or 2, 4, 8, 12, 16 the groove depth below the indicator. In the former case, the groove depth grows in equal steps and in the latter, first by two and after that by four units, advantageously by millimeter. It is certainly more important to receive more specific information about the groove depth the closer the depth is to the permitted lower limit, whereas when a less worn tyre is in question, it is not essential to know the degree of wearing with the same accuracy.
According to an advantageous embodiment of the invention, the deepest indicator is at most as deep as the groove of the tread, but advantageously at least 1 to 2 mm lower than that so that it will give a warning of the small remaining groove depth.
According to an advantageous embodiment of the invention, the indicator capillary grooves are formed at sufficient spaces in the tread so that in the tyre cast phase, rubber mass may enter into all spaces formed by lamellae. A sufficient space between the indicators is approximately equal to the greatest dimension of the cross-section image of individual indicators.
According to an advantageous embodiment of the invention, the tread is preferably provided with two or more identical indicator groups, spread on both sides of the middle line in circumference direction of the tyre and/or approximately at equal spaces on the entire tyre circumference. Hence, one group of indicators is always located in a visible location, irrespective of the fact in which wheel the tyre is located and where the inspector is standing.
According to an advantageous embodiment of the invention, the indicators are Arabic or Roman numerals or numerals of any other numerical system or other graphic signs, symbols or images. Letters or characters can be used as graphic signs. Thereby, the information provided by letters concerning the groove depth can be expressed in words describing the condition of the tyre. The letters forming one word always extend to the same groove depth. The condition of a tyre can be described, for instance with words xe2x80x9cVERY GOOD CONDITIONxe2x80x9d, so that the letters of the word xe2x80x9cVERYxe2x80x9d always extend only a little distance into the tread, and the letters of the words xe2x80x9cGOOD CONDITIONxe2x80x9d extend deep, for instance to a plane in which only 3 mm of the groove depth is left. The letters of xe2x80x9cVERYxe2x80x9d wear at a relatively early stage off so that the tyre indicates then xe2x80x9cGOOD CONDITIONxe2x80x9d.
While in this condition, the tyre can be used for driving for a relatively long time, and it is not until only 3 mm of the groove depth is left, the words xe2x80x9cGOOD CONDITIONxe2x80x9d wear off.
According to an advantageous embodiment, the indicators can be of the shape of W and S. The letters W wear down more rapidly so that the letters S which extend deeper remain. The mere S indicators show that a tyre originally appropriate for winter tyre (W+S) is merely in summer tyre condition.
According to an advantageous embodiment, for the indicator a well-known xe2x80x9csmiling mouthxe2x80x9d image can be used, with a round face, two spots for eyes and a smiling mouth. The indicators are formed from a smiling mouth indicator and a sorry mouth indicator, and possibly also from a straight mouth indicator. The smiling mouth extends only a little bit into the tread so that the smiling condition of the tyre is over after the smiling mouth indicator has worn off from the surface. The sorry mouth indicator and possibly the straight mouth indicator are left there. When the alarming sorry mouth indicator alone is visible, it is time to replace the tyre.
According to an advantageous embodiment, the surface of one piece or preferably of more tread pattern pieces is provided with indicators representing a series of numbers. The series of numbers are advantageously growing from the margin towards the middle line, so that the numbers in the middle, indicating a greater depth, are the first ones to wear off from the surface. Various tread pattern pieces may be provided with the same series or different series of numbers, so that the different series of numbers disappear from the surface at different speeds. When such number series indicators are disposed in adjacent tread pattern pieces in the width direction of the tyre, information is received, not only about groove wear but also about potential uneven wear. Individual numbers are provided advantageously in the form of capillary grooves being mutually apart. Capillary grooves extending across the pattern piece can be provided between rows of numbers.
According to another advantageous embodiment of the invention, the indicators indicating the groove depth have been formed in a groove, in the upper surface of a piece formed between the walls bordering the groove, said upper surface being planar and parallel to the tread.
This is appropriate particularly for winter tyres, the tread pattern pieces of which already have been provided a plurality of capillary grooves for enhancing the grip to the road. Forming additional grooves in tread pattern pieces of a winter tyre may cause that the tyre becomes unstable and, in addition, the tread pattern of the tread would suffer from it. Also making a mould for the tyre becomes more difficult and the casting is not always successful either. When forming indicators in a piece like that in a groove between the tread pattern pieces, the tread pattern shaping of the tread will not suffer therefrom. With the exception of the location, the indicators formed in a groove piece are similar to the indicators described above, being formed in the wear surface.
According to an advantageous embodiment, the piece formed in the groove is only attached on one wall bordering the groove and detached from the opposite wall, which is preferable concerning the grip to the road.
According to an advantageous embodiment, the piece formed in a groove is stepped, which is similarly preferable concerning the grip to the road. One indicator of one indicator group has been formed in each step.
As the indicator acting as a wear warning according to the invention, a series of numbers is preferred which clearly indicates the groove depth remaining in the bottom plane of each capillary groove. If a series of numbers with the highest number 5 can be seen on the tread of a slightly worn tyre, it means that 5 mm or five of other measuring units of groove depth plus the height remaining in the sipe are remaining.
A great advantage of a tyre according to the invention compared with other corresponding structures known in prior art is that the pattern formed by the indicators in the tread can easily be checked, and it will tell directly the condition of the wear surface at each moment by means of measuring figures, words or images.
As a matter of fact, the words can be in any language and in any letters or characters as long as they can be understood to indicate the remaining groove depth of the tread as a measuring figure or in proportion to the acceptable measuring figures determined by legislation.
Indicators in the form of sipes can, according to the invention, be used both in summer and winter tyres. In a summer tyre, they are generally disposed in ribs in circumferential direction, and in a winter tyre, they are fitted in ribs and/or in the surface of tread pattern pieces or in grooves between tread pattern pieces. They are advantageously fitted between other capillary grooves or possibly in conjunction therewith. When the indicators are disposed in winter tyres grooves, in groove pieces located in the grooves, the capillary indicator grooves will not cause any trouble in providing tread pattern pieces with other capillary grooves to increase the grip to the road. When positioning indicators in a tread pattern piece of a winter tyre, they contribute to the grip of the tyre to the road and make it better.
Indicators of different types are provided using the lamination technique of the invention. Lamellae corresponding to indicators are disposed in the segments of a tyre mould so that corresponding indicators in the form of capillary grooves are formed in the finished tyre. The production is simple and the sipe indicators will not increase significantly the tyre production costs.